Candidate: Since graduate school, I have been interested in the homeostatic mechanisms that operate against weight loss for obese and diabetic individuals. As a postdoctoral fellow in the laboratory of Anthony Hollenberg, MD at the Beth Israel Deaconess Medical Center (BIDMC) and Harvard Medical School (HMS), my research has focused on the regulation of thyrotropin-releasing hormone (TRH) expression in the paraventricular nucleus of the hypothalamus (PVH) by both thyroid hormone and pathways important in the regulation of body weight. My work detailing the transcriptional regulation of TRH by thyroid hormone was published in Endocrinology. Recent work on the regulation of thyroid hormone levels by neuropeptide Y through the central thyroid axis and the melanocortin 4 receptor through hepatic thyroid hormone metabolism resulted in a 1st-author publication in Cell Metabolism. In addition, this work has been recognized by Women in Endocrinology and several times by The Endocrine Society. As I transition from postdoctoral fellow to Instructor, I look to target the specific neurons responsible for fasting-induced suppression of TH levels and use the results of this research to apply for extramural funding, such as an R01. My overall career goal is to become a fully independent scientist running a laboratory dedicated to answering fundamental biological questions aimed at human health and disease. The K01 award will give me the time to acquire the skills necessary to achieve this goal including training in specific techniques and the intangibles of becoming a successful independent scientist including grant and manuscript writing, lab management and teaching. Environment: The depth of expertise in the Division of Endocrinology, Metabolism and Diabetes at the BIDMC makes the Division an ideal site for training scientists in molecular physiology and metabolism. The environment at the BIDMC and HMS and its other academic medical centers within the Longwood Medical Area will provide me with outstanding opportunities for training, career development workshops and didactic courses. Importantly, my development activities during the award period include training in research techniques and in the skills necessary to become an independent investigator such as grant and manuscript writing, public speaking, collaboration, lab management and teaching. This training will be accomplished through formal coursework and individual instruction. I have identified a Primary Mentor, Dr. Anthony Hollenberg, a co-Mentor, Dr. Brad Lowell, and an Advisory Committee comprised of Drs. P. Reed Larsen and Terry Maratos-Flier who are committed to my development as an independent investigator. I will meet weekly with Dr. Hollenberg, biweekly with Dr. Lowell and formally three times a year with Drs. Larsen and Maratos- Flier to present hypotheses, experimental designs, results, and future directions. These meetings will incorporate the skill set discussed above and also integrate training in responsible conduct in research and assistance in preparing applications for tenure-track faculty positions at outstanding institutions. Research: Regulation of the hypothalamic-pituitary-thyroid (HPT) axis maintains precise circulating thyroid hormone (TH) levels. The central HPT axis includes thyrotropin-releasing hormone (TRH) release from the paraventicular nucleus (PVN) of the hypothalamus, which stimulates release of thyroid-stimulating hormone (TSH) to maintain normal TH levels. TH levels are integral to several processes including metabolism and energy expenditure. Thus, acute regulation of TH levels during periods of nutritional stress or illness is an important adaptive mechanism for survival. Metabolic signals are necessary to suppress TH levels to curb energy expenditure during a prolonged fast. Neuropeptide Y (NPY) is required for fasting-induced suppression of Trh expression in the PVN, whereas the melanocortin 4 receptor (MC4R) is required for activation of hepatic TH metabolism. Thus, fasting-induced suppression of TH levels is controlled centrally by the HPT axis and peripherally by hepatic TH metabolism. This research proposal seeks to determine the exact subsets of NPY and MC4R neurons responsible for regulation of the HPT axis and hepatic TH metabolism, respectively, and how obesity and illness disrupt this regulation under three aims: 1. To establish that NPY neurons in the arcuate nucleus signal to hypophysiotropic neurons in the PVN to regulate the HPT axis. 2. To ascertain the role of the arcuate nucleus in mediating TH-suppression of Trh expression in the PVN. 3. To discern which subsets of MC4R neurons control TH metabolism by the liver. These aims will be investigated using a combination of knockout mouse models, specialized diets to induce obesity or hypothyroidism, and Designer Receptors Exclusively Activated by Designer Drugs (DREADD) technology. DREADDs are mutant receptors that are stimulated only by the pharmacologically inert ligand clozapine-N-oxide (CNO). These mutant receptors are targeted to specific subsets of neurons using the Cre-LoxP recombination system in transgenic mice. By administering CNO to mice expressing DREADDs in NPY or MC4R neurons, this technology will pinpoint the specific subsets of NPY and MC4R neurons that control TH levels, which is the first time this technology has been used to understand neuroendocrine regulation. This project aims to provide a better understanding of how multiple metabolic signals are integrated to establish the set point of the HPT axis. Ultimately, defining the subsets of neurons that control TH levels will allow for the identification of new and specific therapeutic targets for the treatment of metabolic disorders and other diseases while also assisting in maintaining long-term weight loss.